Method for forming a cap without draft allowance for a pneumatic gun

ABSTRACT

A method has the steps of forming a metal mold with a specific scale, feeding molten material with a desired temperature into the cylinder, injecting the molten material into the impression at a specific speed, and cooling the molten material to solidify. With such a method, a cap without draft allowance is formed, such that the process for forming the cap is simple and the cost for manufacturing the cap low.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method, and more particularlyto a method for forming a cap without any draft allowance during a diecasting process.

[0003] 2. Description of Related Art

[0004] A cap is always mounted on a pneumatic gun to define an airchamber for the piston of the pneumatic gun. To achieve a good precisionin size, the cap is always formed with the cold chamber method of a diecasting process. A metal mold is used to form the cap and has animpression with a shape substantially equal to that of the cap anddefined in the mold. To conveniently take the cap out from the mold,draft allowances are previously set in the impression of the metal mold.

[0005] However, oblique faces are formed in the cap due to the draftallowances in the mold. After the cap is formed with the die castingprocess, the unnecessary portion formed by the draft allowances must becut off. An additional task must be performed on the cap, such that theprocesses for forming the cap for a pneumatic gun are troublesome andthe cost for manufacturing the cap is high.

[0006] To overcome the shortcomings, the present invention tends toprovide a method for forming a cap without draft allowance to mitigateand obviate the aforementioned problems.

SUMMARY OF THE INVENTION

[0007] The main objective of the invention is to provide a method forforming a cap without draft allowances during the die casting process.The method has the steps of forming a metal mold with a specific scale,feeding molten material at a desired temperature into the cylinder,injecting the molten material into the impression at a specific speed,and cooling the molten material to solidify. With such a method, a capwithout draft allowance is formed, such that the process for forming thecap is simplified and the cost for manufacturing the cap is reduced.

[0008] Other objects, advantages and novel features of the inventionwill become more apparent from the following detailed description whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a cross sectional side plan view of a cap formed with amethod in accordance with the present invention; and

[0010]FIG. 2 is a side plan view in partial cross section of a metalmold for forming the cap in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0011] With reference to FIG. 1, a cap (30) for a pneumatic gun inaccordance with the present invention comprises a body with a closedfirst end wall and a second end wall. An opening is defined in thesecond end wall of the body and has a floor parallel to an edge of thesecond end wall of the body. An annular flange (31) perpendicularlyextends from the floor in the opening. No draft allowance is arranged inthe cap (30) as will be described in more detail later.

[0012] To form the cap (30) without draft allowances, with reference toFIGS. 1 and 2, a metal mold constructed of a female portion (10), a maleportion (20) and a cylinder (11) is provided. An impression (100) with ashape substantially equal to that of the cap (30) is defined between theportions (10,20). Multiple cooling channels (13) are defined in theportions halves (10,20) and extend around the impression (100). Theportions (10,20) are selectively made of SKD 61 alloy tool steel. Thehardness of the portions (10,20) is in a range H_(RC) 45 to 50. A heattreatment must be applied to the surface of the portions (10,20) toincrease the hardness of the surface of the portions (10,20) to H_(RC)70 with a depth 0.25 mm (millimeter). The smoothness of the surface ofthe portions (10,20) is 800 μ.

[0013] The cylinder (11) is connected to one of the portions (10,20) andis communicated with the impression (100) through an injecting passage(102). A piston (12) is moveably mounted in the cylinder (11) to forcemolten material such as Aluminium into the impression (100). Thepreferred temperature of the molten Aluminum is 650 to 680° C. A sprue(110) is defined in the cylinder (11) for the molten material to be fedinto the cylinder (11).

[0014] To inject the molten material into the impression (100), thepiston (12) presses the molten material into the injecting passage (102)at a low speed 1.5 m/s (meters per second) firstly to avoid the air ofthe environment from being sucked into the cylinder (11) through thesprue (110). The molten material is then forced into the impression(100) at a speed 4.5 m/s. The air in the impression (100) will be forcedinto an air chamber (101) communicating with the impression (100). Inpractice, the air chamber (101) is connected to an air pump (not shown)to exhaust the air from the impression (100) and the air chamber (101),such that this can avoid gas bubbles forming in the cap (30). Thepressure provided by the piston (12) to the molten material is increasedto 220 kg/cm² so that the density of the molten material is increased.The molten material is cooled to between 180 and 200° C. in 20 to 25seconds due to cool water passing along the cooling channels, wherebythe molten material solidifies as the cap (30). When the cap (30) hascooled and its volume decreased in comparison to its hot state, a gapwill be defined between the cap (30) and the inner surface of theimpression (100) due to the contraction of the cap (30). The gap canhelp the cap (30) to be ejected from the mold without draft allowance.Thus, the cap (30) can be directly mounted on a pneumatic gun withoutany additional work. Accordingly, the process for manufacturing the cap(30) is simple, and the cost for manufacturing the cap (30) is low. Inaddition, multiple pins (21) are mounted in the mold to push the cap(30) to release it from the mold.

[0015] In addition, to improve the convenience of releasing the cap (30)from the mold, an isolating agent is previously sprayed onto the innersurface of the impression (100) before the die casting process isundertaken. The agent is a mixture of 10% ester, 4% vegetable oil, 6%silicon oil, 1% polymer and 79% water. The agent is diluted with waterat a ratio 1:120 when the agent is used.

[0016] Even though numerous characteristics and advantages of thepresent invention have been set forth in the foregoing description,together with details of the structure and function of the invention,the disclosure is illustrative only, and changes may be made in detail,especially in matters of shape, size, and arrangement of parts withinthe principles of the invention to the full extent indicated by thebroad general meaning of the terms in which the appended claims areexpressed.

What is claimed is:
 1. A method for forming a cap without draftallowance comprising the steps as follow: forming a metal moldconstructed of a female portion, a male portion and a cylinder, animpression with a shape substantially equal to that of the cap anddefined between the portions and the cylinder connected to one of theportions and communicated with the impression through an injectingpassage, wherein each portion has a hardness in a range H_(RC) 45 to 50,a heat treatment is applied to a surface of each portion to increase ahardness of the surface of the portion to H_(RC) 70 with a depth 0.25 mmand a smoothness of the surface of the half is 800 μ; feeding moltenmaterial into the cylinder, wherein the molten material has atemperature between 650 and 680° C.; injecting the molten material intothe impression, wherein the cylinder forces the molten material into theinjecting passage at a low speed 1.5 m/s (meter per second) firstly, themolten material is then forced into the impression at a speed 4.5 m/s,and the cylinder provides a pressure about 220 kg/cm² to the moltenmaterial to increase the density of the molten material; and cooling themolten material with cool water to solidify the molten material, whereinthe molten material is cooled to between 180 and 200° C. in 20 to 25seconds, and the molten material solidifies to a cap with a desiredshape without draft allowance.
 2. The method as claimed in claim 1further comprising spraying an isolating agent onto an inner surface ofthe impression before the molten material is fed into the cylinder. 3.The method as claimed in claim 2, wherein the agent is a mix of 10%ester, 4% vegetable oil, 6% silicon oil, 1% polymer and 79% water; andthe agent is diluted with water at a ratio 1:120 when the agent is used.4. The method as claimed in claim 1, wherein the mold has an air chambercommunicating with the impression; and the air chamber is connected toan air pump to exhaust air from the impression and the air chamber whenthe molten material is forced into the impression.
 5. The method asclaimed in claim 1, wherein each portion of the metal mold is made ofSKD 61 alloy tool steel.